W. T. Hill | USGS Science

USGS1971

An early middle ordovician age for collapse breccias in the east Tennessee zinc districts as indicated by compaction and porosity features

The carbonate strata of the Lower Ordovician formations in the southern Appalachian Valley were probably considerably thicker, less compacted, and more porous and permeable in early Middle Ordovician time than they are now. Analysis of selected compaction features of rocks associated with the breccias of the East Tennessee zinc districts permits quantitative estimation of the relative compaction (reduction in thickness) these sediments have undergone before and after brecciation. Total reduction in thickness of certain Lower Ordovician marker beds because of compaction during burial under younger Paleozoic sedimentary rocks and imbricate Appalachian thrust sheets appears to be at least some 40 to 50 percent. Furthermore, it is also estimated that about half of this compaction took place after brecciation in the solution-collapse structures of the zinc deposits. In comparison with laboratory compaction tests on carbonate muds it is apparent that the Lower Ordovician rocks had been buried to a depth of at least several thousand feet by the end of the Paleozoic Era. On the other hand the zones of collapse breccias probably formed at depths of generally less than a thousand feet, thus indicating an early Middle Ordovician age for their development. The general character of the rock during this time probably was similar to that of Tertiary limestone, which acts as the principal aquifer in the Coastal Plain region of southeastern United States.

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USGS1971

Problems on the origin of ore deposits in the lower Ordovician formations of east Tennessee

Most recent workers in the East Tennessee zinc and barite districts are in general agreement that the host breccias were formed by solution-collapse processes in early Middle Ordovician time, probably in an ancient carbonate aquifer system of regional extent. There is little agreement, however, on the source and nature of the ore fluids from which the epigenetic minerals were formed. In this symposium Hanshaw and his colleagues have presented evidence in support of the concept that much secondary dolomite may have been formed by ground water. On the other hand, Roedder has shown that fluid inclusion studies indicate hot saline brines (75 degrees -150 degrees C) as the source fluids for the ore minerals, and he suggests that they may have been deeply circulating connate brines. Other investigators in the past have proposed a variety of source fluids, including juvenile waters, meteoric waters that have become enriched by the leaching of particular elements from the surrounding rock, and sea water. In the zinc deposits, sphalerite is accompanied locally by minor amounts of galena and pyrite, and occasionally, traces of chalcopyrite. Dolomite is the chief gangue mineral and is associated with lesser amounts of quartz, calcite, barite, fluorite, anhydrite, gypsum, and bitumen. Some deposits containing barite as the dominant ore mineral have various amounts of fluorite and pyrite; sphalerite and galena may be present locally along with the other gangue minerals typical of the zinc deposits. The paragenetic sequences of the minerals have been determined for most of the mines and many of the prospects, and, except for minor refinements, will be little changed by additional studies. The real problem, though, is the correlation of these depositional sequences from mine to mine and from district to district. Perhaps the major difficulty in the correlation is the seemingly uncomplicated mineralogy that disguises a rather complex regional paragenesis. Questions regarding the routes the ore fluids travelled to reach the sites of deposition, the reasons why the ores are concentrated in some areas more than others, and the reasons why the mineralogy varies from area to area are questions that are unanswered or at best only partly answered. Studies to date have produced convincing evidence concerning the nature of the structures and the physical conditions responsible for the localization of ore in the Kingsport Formation (Lower Ordovician). On the other hand, the relation of the Kingsport ores to the smaller deposits in the underlying Lower Ordovician and Cambrian rocks is but little understood, and no final answer to the origin of these deposits can be reached until this relationship has been satisfactorily explained. Fluid inclusion studies, trace element analysis, and isotopic studies of all sorts are needed to further our understanding of these seemingly simple ores. Continued detailed studies of field relations, both regional and local, are of major importance, not only for what they will reveal in themselves, but also to perfect the frames of reference in which to place the growing mass of laboratory data.

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